This project is evaluating the associations between individual differences in platelet monoamine oxidase activity and human behavior. Observations originally reported by this group in 1972 that patients with bipolar affective disorders and chronic schizophrenia have reduced platelet MAO activity, while patients with unipolar affective disorders and acute schizophrenia do not, have been substantiated in studies of larger numbers of patients and over 500 normal controls. In investigations of factors which affect MAO activity, genetic contributions, sex differences, sex steroids and certain drugs have been shown to affect MAO activity in normal human and psychiatric patient populations and in rhesus monkeys. Correlations between platelet monoamine oxidase activity and individual differences in spontaneous behavior in the rhesus monkey and in human personality characteristics measured by psychological testing have also been observed.